Collagen foam dressings have been made heretofore by a number of processes and have been found to be advantageous as skin dressings or skin plasters particularly suitable for the treatment of burns (see British Pat. No. 942,226).
In one prior-art technique a 1.2% by weight aqueous solution of collagen is homogenized and introduced into a cylinder having a fritted-glass bottom through which air or another gas is introduced under pressure to foam the collagen. The collagen is then gelled by treating it with a cadmium nitrate solution, the cadmium nitrate solution being washed out of the gel product. The result is a porous collagen foam which may be used effectively as a bandage and may include a bacteriocidal or a bacteriostatic agent. Because this collagen gel is friable and sensitive to mechanical breakdown, it is frequently covered with a nonfoamed collagen foil of tougher character. The disadvantage of this system is that it requires a relatively large number of steps and a foil overlay dense enough to obstruct secretion from the wound and hence interfere with an advantageous property of the porous collagen layer.
In another system for the formation of collagen dressings, a spongy product can be produced by freezing a collagen sol for at least 12 hours at -20.degree.C and then eliminating the water by contacting the frozen sol with an organic solvent having an affinity for water and which does not attack the collagen. The latter treatment (water removal) is carried out in three or four successive extractions which together require about 8 hours for removal of water to the point that the product has the required spongy character. The sponge layer has a density of 0.015 to 0.6 gram/cm.sup.3, is of good porosity and is a highly desirable bandage or dressing material. However, the process for making it is laborious and takes a relatively long time.
It has also been proposed to freeze an acidic collagen sol at a temperature close to the freezing point of the aqueous system and to subject the system to a high vacuum so that water is removed by sublimation. This technique has the disadvantage that it yields a product having large pores and hence little or no utility for dressing material.